1. Field of the Invention
The present invention relates to an optical attenuator having two Faraday rotators, and more particularly to an optical attenuator and a system, optical amplifier, and terminal device each having the optical attenuator.
2. Description of the Related Art
In constructing an optical communication system, there is a case that an optical attenuator is used to adjust the power of light to be supplied to an optical device such as an optical amplifier. In an example of this kind of optical attenuator, attenuation is changed by mechanical operation. For example, an attenuator film having an attenuation distribution is inserted in an optical path, and is mechanically displaced to thereby adjust attenuation.
In some case, it is practically required to incorporate an optical attenuator in a system such that the attenuation by the optical attenuator is controlled. For example, in an erbium-doped fiber amplifier (EDFA), an optical signal once amplified is supplied to an optical attenuator for giving an attenuation feedback-controlled by a monitored value of output level, thereby maintaining a constant output level. In such a case, the use of an optical attenuator whose attenuation is mechanically adjusted is not preferable in improving reliability of the system.
In view of this fact, the present inventor has already proposed a practically excellent optical attenuator having no mechanical movable parts (Japanese Patent Application No. 4-205044, for example). This optical attenuator has a Faraday rotator whose Faraday rotation angle changes with a change in current applied to an electromagnet, and the attenuation is set by adjusting the Faraday rotation angle.
It is desirable that an optical attenuator gives a uniform attenuation to input light irrespective of its wavelength. However, in the case that the Faraday rotator has a wavelength characteristic, i.e., in the case that the Faraday rotation angle changes depending upon wavelength, the attenuation changes according to wavelength, so that the wavelength characteristic of attenuation becomes nonflat. If an optical attenuator having a nonflat wavelength characteristic of attenuation is applied to a wavelength division multiplex system, the attenuations of optical signals become different between channels, causing a problem such as interchannel deviation in signal power.
Further, it is also required that the wavelength characteristic of attenuation of an optical attenuator can be arbitrarily set to cancel gain tilt (a property of change of gain according to wavelength) occurring in an EDFA, for example.
It is therefore an object of the present invention to provide an optical attenuator having a flat wavelength characteristic of attenuation.
It is another object of the present invention to provide an optical attenuator having an adjustable wavelength characteristic of attenuation.
It is a further object of the present invention to provide a system, optical amplifier, and terminal device each having such an optical attenuator.
In accordance with a first aspect of the present invention, there is provided an optical attenuator comprising first and second attenuator units cascaded on an optical path, and a control circuit connected to the first and second attenuator units. Each of the first and second attenuator units includes a Faraday rotator provided on the optical path. The Faraday rotator generates a Faraday rotation angle given as a function of wavelength. Each of the first and second attenuator units further includes polarizing means for generating an attenuation determined by the Faraday rotation angle on the optical path. Particularly in the optical attenuator according to this aspect of the present invention, the control circuit includes means for controlling the Faraday rotation angle in each of the first and second attenuator units so that a wavelength characteristic of attenuation in the first attenuator unit is substantially canceled by a wavelength characteristic of attenuation in the second attenuator unit.
With this configuration, although each Faraday rotator generates a Faraday rotation angle given as a function of wavelength, a wavelength characteristic of total attenuation is substantially flattened by the operation of the control circuit.
In an optical attenuator in accordance with a second aspect of the present invention, the control circuit includes means for controlling the Faraday rotation angle in each of the first and second attenuator units so that a wavelength characteristic given as the sum of a wavelength characteristic of attenuation in the first attenuator unit and a wavelength characteristic of attenuation in the second attenuator unit has a desired tilt. With this configuration, the wavelength characteristic of total attenuator can be freely adjusted.
In accordance with a third aspect of the present invention, there is provided an optical attenuator comprising first and second attenuator units each for giving a variable attenuation; the first and second attenuator units having first and second wavelength characteristics of attenuation, respectively, which are different from each other.
In accordance with a fourth aspect of the present invention, there is provided a system comprising an optical transmission line for transmitting wavelength division multiplexed light including a plurality of optical signals having different wavelengths; and first and second attenuator units arranged along the optical transmission line, each for giving a variable attenuation; the first and second attenuator units having first and second wavelength characteristics of attenuation, respectively, which are different from each other.
In accordance with a fifth aspect of the present invention, there is provided an optical amplifier comprising a first optical amplifying unit; a second optical amplifying unit; and first and second attenuator units provided between the first and second optical amplifying units, each for giving a variable attenuation; the first and second attenuator units having first and second wavelength characteristics of attenuation, respectively, which are different from each other.
In accordance with a sixth aspect of the present invention, there is provided an optical amplifier comprising an optical amplifying unit; and an optical attenuator connected to an output of the optical amplifying unit; the optical attenuator comprising first and second attenuator units each for giving a variable attenuation; the first and second attenuator units having first and second wavelength characteristics of attenuation, respectively, which are different from each other.
In accordance with a seventh aspect of the present invention, there is provided an optical amplifier comprising an optical amplifying unit; and an optical attenuator connected to an input of the optical amplifying unit; the optical attenuator comprising first and second attenuator units each for giving a variable attenuation; the first and second attenuator units having first and second wavelength characteristics of attenuation, respectively, which are different from each other.
In accordance with an eighth aspect of the present invention, there is provided a terminal device comprising a plurality of E/O converters for respectively outputting optical signals having different wavelengths; a plurality of level adjusting units for respectively adjusting the levels of the optical signals output from the E/O converters; and an optical multiplexer for wavelength division multiplexing the optical signals output from the level adjusting units to obtain wavelength division multiplexed light; each of the level adjusting units comprising first and second attenuator units each for giving a variable attenuation; the first and second attenuator units having first and second wavelength characteristics of attenuation, respectively, which are different from each other.
In accordance with a ninth aspect of the present invention, there is provided a terminal device comprising a plurality of E/O converters for respectively outputting optical signals having different wavelengths; a plurality of level adjusting units for respectively adjusting the levels of the optical signals output from the E/O converters; an optical multiplexer for wavelength division multiplexing the optical signals output from the level adjusting units to obtain wavelength division multiplexed light; and an optical amplifier for amplifying the wavelength division multiplexed light output from the optical multiplexer; the optical amplifier comprising an optical amplifying unit and an optical attenuator connected to the optical amplifying unit; the optical attenuator comprising first and second attenuator units each for giving a variable attenuation; the first and second attenuator units having first and second wavelength characteristics of attenuation, respectively, which are different from each other.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.